This invention claims priority to prior Japanese patent application JP 2002-147505, the disclosure of which is incorporated herein by reference.
This invention relates to an anode member for a solid electrolytic capacitor, a method of producing the same, and a solid electrolytic capacitor using the same and, in particular, to a flat anode member including a thin plate of a valve metal as an anode lead and a sintered member laminated thereon, a method of producing the same, and a solid electrolytic capacitor using the same.
There is known a solid electrolytic capacitor including an anode including a sintered member formed by sintering a powder of a valve metal such as tantalum (Ta). Typically, the sintered member has a cylindrical shape such as a circular cylinder or a rectangular cylinder. For convenience of description, the capacitor of the type will be referred to as a cylindrical element capacitor. On the other hand, Japanese Unexamined Patent Publication No. S59-219923 (JP 59-219923 A) discloses a capacitor using a sintered member of a flat structure. The capacitor includes a thin plate (foil) of a valve metal and a layered sintered member laminated on the thin plate. For convenience of description, the capacitor of the type will be referred to as a foil element capacitor. This invention relates to the sintered member of the foil element capacitor. Hereinafter, the foil element capacitor will be described in conjunction with a tantalum solid electrolytic capacitor by way of example. It is well known that, in a solid electrolytic capacitor using a sintered member formed by sintering a powder of a valve metal, the sintered member electrically serves as an anode of the capacitor. In this connection, the thin plate of the valve metal and the sintered member formed thereon may collectively be called an anode member in the following description. Japanese Unexamined Patent Publication No. 2002-50550 (JP 2000-50550 A) discloses a method of producing an anode element for a tantalum electrolytic capacitor, in which a paste containing metal powder is applied or printed to form an anode element.
Referring to FIG. 1A, a related tantalum foil element solid electrolytic capacitor will be described. The related tantalum foil element solid electrolytic capacitor includes a tantalum foil 1, i.e., a foil of a tantalum metal and a layered sintered member 2 obtained by sintering a tantalum powder as a material powder and laminated on the tantalum foil 1. The layered sintered member 2 of the tantalum powder has small pores formed by sintering and interconnected in a complicated manner. Therefore, the layered sintered member 2 has a very large surfaced area. As described above, the sintered member 2 serves as an anode. On an outer surface of the sintered member 2 and on inner walls of the small pores, a tantalum oxide (Ta2O5) film (not shown) is formed. The tantalum oxide film serves as a dielectric member of the capacitor. On the tantalum oxide film, a solid electrolyte layer (not shown) is formed. The solid electrolyte layer serves as a cathode of the capacitor. A combination of the sintered member 2 as the anode, the tantalum oxide film as the dielectric member, and the solid electrolyte layer as the cathode forms a fundamental structure of the capacitor.
On the solid electrolyte layer, a conductive substance layer (cathode conductor layer) is formed although not shown in the figure. The cathode conductor layer includes a plurality of layers, for example, a graphite layer and a silver paste layer, successively laminated. To the outermost layer of the cathode conductor layer, a cathode-side terminal (external cathode terminal) 3 for electrical connection to an external circuit is fixedly attached. On the other hand, the tantalum foil 1 partially has an exposed surface on which the sintered member 2 is not formed, as shown at a left side in the figure. To the exposed surface, an anode-side terminal (external anode terminal) 4 for electrical connection to the external circuit is fixedly attached.
An outer resin member 5, for example, made of epoxy resin covers the tantalum foil 1, the layered sintered member 2, and the external cathode and the external anode terminals 3 and 4 except a part of each of the external cathode and the external anode terminals 3 and 4. The part of each of the external cathode and the external anode terminals 3 and 4 led out of the outer resin member 5 is shaped and bent to extend at first along a side wall and then along a bottom surface of the outer resin member 5.
In the foil element solid electrolytic capacitor having the above-mentioned structure, the tantalum foil 1 serves to electrically connect the sintered member 2 as the anode of the capacitor and the external anode terminal 4. Thus, the tantalum foil 1 corresponds to a tantalum wire well known as a so-called xe2x80x9canode leadxe2x80x9d in the cylindrical element solid electrolytic capacitor and planted to the cylindrical sintered member.
The tantalum foil element solid electrolytic capacitor mentioned above is generally produced in the following manner. At first, a tantalum powder (a powder of a tantalum metal), a solvent, and a binder are mixed to form a tantalum powder paste. The solvent and the binder are appropriately selected with respect to each other. For example, a water soluble binder is selected for use with a water-based solvent.
Next, on the tantalum foil 1 separately prepared, the tantalum powder paste is printed to form a tantalum powder layer. As a printing mask, a screen mask or a metal mask may be used. In order to reduce a printing thickness, the screen mask is preferable. In order to increase the printing thickness, the metal mask is appropriate.
Then, the tantalum foil 1 with the tantalum powder layer formed thereon is sintered in a high vacuum of, for example, about 10xe2x88x926 Torr at a temperature lower than the melting point of the tantalum metal, for example, at a high temperature between about 1300xc2x0 C. and about 1600xc2x0 C. Thus, an anode member is obtained.
Thereafter, in the manner similar to the production of the cylindrical element solid electrolytic capacitor, the tantalum oxide film as a dielectric film, the solid electrolyte layer, and the cathode conductor layer are formed. Then, the external cathode and the external anode terminals 3 and 4 are fixedly attached and electrically connected. Furthermore, the outer resin member 5 is formed and the external cathode and the external anode terminals 3 and 4 are shaped.
Specifically, on the inner and the outer surfaces of the anode member obtained by the above-mentioned sintering, a tantalum oxide (Ta2O5) film, i.e., a thin film of oxide of the tantalum metal as a raw material of the sintered member 2 is formed by anodic oxidation well known in the art. Furthermore, on the tantalum oxide film, the solid electrolyte layer is formed. As a solid electrolyte, use may be made of manganese dioxide obtained by thermal decomposition of manganese nitrate or a conductive polymer such as polypyrrole. In recent years, the conductive polymer is increasingly used as the solid electrolyte because the conductive polymer is smaller in intrinsic resistance so that the capacitor is reduced in equivalent series resistance (ESR) and because a heat insulating reaction is quick so that the capacitor hardly emit smoke or catch fire.
Following the formation of the solid electrolyte layer, the cathode conductor layer is formed. Generally, the cathode conductor layer has a laminate structure including the graphite layer formed on the solid electrolyte layer and the silver paste layer formed on the graphite layer. The cathode conductor layer serves to electrically connect the solid electrolyte layer and the external cathode terminal 3. Furthermore, the cathode conductor layer also serves to protect the dielectric film by relaxing the stress which would be produced during formation of the outer resin member 5 in the subsequent production process and upon mounting the capacitor after it is completed. Between the step of forming the solid electrolyte layer and the step of forming the cathode conductor layer, the tantalum oxide film may be re-formed if necessary. Such re-formation is intended to repair a minor defect caused in the tantalum oxide film due to mechanical and chemical stresses produced during formation of the solid electrolyte layer to thereby achieve a more stable characteristic of the capacitor. The reformation is carried out in the manner substantially similar to the formation of the tantalum oxide film mentioned above. Specifically, the anode member is again applied with an electric voltage in an electrolyte solution.
After formation of the cathode conductor layer, the external cathode terminal 3 is fixedly attached and electrically connected to the cathode conductor layer, for example, by adhesion using a conductive adhesive. To the exposed surface of the tantalum foil 1 which is not covered with the sintered member 2, the external anode terminal 4 is fixedly attached by welding or the like.
Finally, the outer resin member 5 is formed by transfer molding using thermosetting resin such as epoxy resin. The external cathode and the external anode terminals 3 and 4 are shaped as mentioned above. Thus, the related tantalum foil element solid electrolytic capacitor illustrated in FIG. 1A is completed.
As compared with the cylindrical element solid electrolytic capacitor, the above-mentioned foil element solid electrolytic capacitor is advantageous in the following respects. Specifically, the thickness of the anode member can easily be reduced so that the capacitor can advantageously be reduced in thickness and size. Since a contact area between the tantalum foil 1 as the anode lead and the layered sintered member 2 as the anode is increased and the resistance therebetween is reduced, the ESR of the capacitor can be lowered.
The related foil element solid electrolytic capacitor illustrated in FIG. 1A has a characteristic which will presently be described.
Referring to FIG. 1B, illustration is made of a related tantalum foil-type anode member used in the capacitor illustrated in FIG. 1A. The layered sintered member 2 of the anode member has a structure shown in an enlarged view encircled by a broken line. Specifically, the layered sintered member 2 is made of a single kind of material powder which is prepared from a single kind of metal (tantalum in the illustrated example) and which has a single kind of average particle size. The thin plate of the valve metal (tantalum foil 1 in the illustrated example) serving as the anode lead is made of a metal (tantalum in the illustrated example) same as a raw metal material of the material powder of the sintered member 2.
As compared with the cylindrical element solid electrolytic capacitor, the foil element solid electrolytic capacitor has the above-mentioned advantages. On the other hand, the foil element solid electrolytic capacitor is disadvantageous in that production is difficult as compared with the cylindrical element solid electrolytic capacitor. Hereinafter, the disadvantage will be described.
As described above, the anode member of a foil type is obtained by depositing (for example, by printing) the tantalum powder paste on the tantalum foil 1 as the valve metal to form the tantalum powder layer and then sintering the tantalum powder layer.
In the meanwhile, sinterability upon sintering a metal powder, i.e., the degree of coupling or adhesion between particles forming the powder or the degree of growth of the particles is widely different depending upon locations of the particles. Specifically, the sinterability at a boundary between the particles is different from the sinterability at an interface between the particles and a macroscopic metal object such as a metal foil or a metal plate. It is known that, even at a same temperature, sintering is quick at the interface between the particles while the growth or the adhesion of the particles is difficult or slow between the particles and the metal foil. It is assumed here that, in the related foil type anode member illustrated in FIG. 1B, the sintering temperature is determined focusing upon the porosity of the sintered member 2. Then, an excellent sintered condition is obtained in a region occupied by the tantalum powder (a main body of the sintered member 2). On the other hand, at the interface between the tantalum foil 1 and the sintered member 2, sintering is insufficient so that adhesion or bond between the sintered member 2 and the tantalum foil 1 is weak. During handling in a production process, the sintered member 2 may often be separated or released from the tantalum foil 1. In addition, the capacitor is deteriorated in leakage current characteristic.
In order to avoid occurrence of the above-mentioned trouble during the production process and the deterioration in characteristic of the capacitor as a result of focusing upon the porosity of the sintered member 2, the sintering temperature is elevated so as to enhance the adhesion or the bond at the interface between the tantalum foil 1 and the powder layer (sintered member 2). In this event, however, over-sintering occurs in the main body of the sintered member 2 so that the porosity is lost and the capacitance of the capacitor per unit weight is decreased. Such over-sintering of the sintered member 2 results in deformation, such as warp, of the anode member and easy occurrence of shape error.
On the other hand, the cylindrical anode member does not use the foil but uses a metal wire as the anode lead. In the cylindrical anode member, the anode lead is enclosed in the tantalum powder. In addition, the tantalum powder is pressed and formed into a cylindrical body by compression molding under pressure. Therefore, contact between the anode lead and the tantalum powder is enhanced. Thus, at an initial stage prior to sintering, the bond between the anode lead and the tantalum powder is already strengthened. From the above-mentioned reasons, strong bond is achieved between the anode lead and the sintered member as compared with the foil-type electrode member even if the sintering is carried out at the temperature determined focusing upon the porosity, i.e., at a relatively low temperature.
Therefore, it is an object of this invention to provide a foil-type anode member for a solid electrolytic capacitor, which includes a thin plate of a valve metal as an anode lead and a layered sintered member obtained by forming on the thin plate a powder layer of a valve metal powder as a material powder to be sintered and then sintering the powder layer and which is improved in bonding strength between the sintered member and the thin plate as the anode lead in case where sintering is carried out at a temperature suitable for porosity of the sintered member.
According to this invention, there is provided an anode member for a solid electrolytic capacitor, comprising a thin plate of a valve metal and a layered sintered member obtained by forming on the thin plate a powder layer of a valve metal powder as a material powder to be sintered and then sintering the powder layer, wherein the layered sintered member has a multi-layer structure including a plurality of sintered layers different in material powder and in sintered condition.
In the layered sintered member having a multi-layer structure, the sintered layers include a heavily-sintered layer and a lightly-sintered layer as a lower layer and an upper layer, respectively.